Fuel cell optimum operation point tracking system in power supply device using fuel cell and power supply device provided with this fuel cell optimum operation point tracking system

ABSTRACT

The present invention provides a fuel cell optimum operating point tracking system capable of ensuring optimum operation of a fuel cell, by detecting the optimum operation voltage of the fuel cell taking not only temperature dependence of the output characteristics thereof, but also chemical reactions into consideration. The fuel cell optimum operating point tracking system is configured so as to vary a voltage of a fuel cell ( 1 ) output upon activation of a power source device ( 2 ) up to as high as a maximum voltage for the maximum power point tracking control by the fuel cell output voltage variation command unit ( 11 ), to measure the power state using a fuel cell output power measuring unit ( 12 ), to monitor the output power measured by the fuel cell output power measuring unit using a fuel cell maximum power point judging unit ( 13 ) to thereby judge the maximum power point of the output voltage of the fuel cell, and to track an optimum operating point through maximum power monitoring to thereby keep the power source operation constant at a stable condition using an optimum operating point variation command unit ( 15 ), and additionally giving a minimal voltage change at around the current operating voltage value.

FIELD OF THE INVENTION

The present invention relates to a power source device for obtaining adesired power of a fuel cell, and in particular to a fuel cell optimumoperating point tracking system used for the power source device, makingit possible to constantly supply a maximum output power from the fuelcell even under variable external environment, such as temperature,around the site where the fuel cell is installed.

RELATED ART

There is known a conventional optimum operating point tracking circuitof a DC/DC converter, typically as shown in FIG. 20, aiming at supplyinga large as possible electric power corresponding to power generationstate of the fuel cell, which uses a fixed voltage arbitrarily set whilepreliminarily predicting so that the output voltage of a fuel cell DMFC,connected as an input power source of the device, can be maximized (seePatent Document 1, for example).

-   -   [Patent Document 1] Japanese Laid-Open Patent Publication        “Tokkaihei” No. 11-341699

DISCLOSURE OF THE INVENTION Subjects to be Solved by the Invention

In this case, it is however impossible to track the optimum operationvoltage for the maximum power supply which shifts in the positivedirection due to ambient temperature of the fuel cell or heat generationin association with the power generation.

It is also typically anticipated that a plurality of maximum power peakvalues may be permissible, because state of power generation by the fuelcell largely varies depending not only on temperature, but also on, forexample, chemical reaction of a fuel used for the fuel cell. In thiscase, the power peak with respect to load current may vary, and this mayraise a problem in that any initial point of observation will no more bea maximum power peak, so that an observed peak value may bemisunderstood as the maximum power, despite a real maximum power resideselsewhere in other peak.

The present invention was conceived considering the above-describedproblems, and is to provide a power source device using a fuel cell,capable of ensuring an optimum operation of the fuel cell by detectingthe optimum operation voltage taking not only temperature dependence ofthe output characteristics, but also chemical reactions intoconsideration.

SUMMARY OF THE INVENTION

According to the present invention aimed at solving the above-describedproblems, there is provided a fuel cell optimum operating point trackingsystem used in a power source device powered by a fuel cell, configuredso as to improve the responsiveness thereof by monitoring the powerstate while varying the output voltage of the fuel cell, and by allowingthe fuel cell to start operation at an input voltage corresponded to amaximum power point thereof, comprising: a fuel cell maximum powersearch function; and a fuel cell optimum operating point trackingfunction capable of tracking an optimum operating point through maximumpower monitoring, by allowing the fuel cell maximum power searchfunction to operate so as to monitor the power state to thereby keep thepower source operation constant at a stable condition, and additionallygiving a minimal voltage change at around the current operating voltagevalue.

The fuel cell maximum power search function comprises a fuel cell outputvoltage variation command unit capable of varying the output voltage ofthe fuel cell upon activation thereof up to as high as the maximumvoltage for the maximum power point tracking control.

The fuel cell output voltage variation command unit is configured ashaving a maximum command voltage and a minimum command voltage settherein, and so as to vary an initial command voltage output uponactivation of the fuel cell up to the maximum command voltage, and so asto further vary the maximum command voltage down to the minimum commandvoltage.

The fuel cell maximum power search function comprises a fuel cell outputpower measuring unit capable of measuring the power state of the fuelcell by varying the output voltage upon activation thereof.

The fuel cell maximum power search function comprises a fuel cellmaximum power point judging-and-storing unit capable of monitoring theoutput power of the fuel cell upon activation thereof, and of judgingthe maximum power point of the output voltage of the fuel cell. The fuelcell maximum power point judging-and-storing unit is configured so as tojudge a voltage under which the command voltage becomes maximum within arange of the successive detection as the maximum power point, when themaximum power point is successively detected despite the command voltageis varied.

The fuel cell maximum power search function comprises a timer having aspecified voltage refreshing interval set therein, configured so as tomeasure the power state by clearing, after the elapse of the specifiedvoltage refreshing interval, the stored values of the fuel cell outputvoltage maximum point and the output voltage corresponded thereto uponactivation of the fuel cell, and by varying the output voltage of thefuel cell up to as high as the maximum voltage for the maximum powerpoint tracking control.

In another configuration, the fuel cell maximum power search functioncomprises a power source activation detecting unit capable of measuringthe power state by varying the output voltage of the fuel cell uponactivation thereof up to as high as the maximum voltage for the maximumpower point tracking control.

The fuel cell maximum power search function comprises an optimumoperating point variation command unit capable of tracking the optimumoperation point, by monitoring the power state at the fuel cell outputpower maximum point upon activation of the fuel cell, and by effectingthe maximum power monitoring through additionally giving a minimalvoltage change at around the current operating voltage value.

The fuel cell optimum operating point tracking function comprises a fuelcell output voltage variation command unit capable of varying the outputvoltage upon being activated by the fuel cell maximum power searchfunction up to as high as the maximum voltage for the maximum powerpoint tracking control.

The fuel cell optimum operating point tracking function comprises a fuelcell output power measuring unit capable of measuring the power state byvarying the output voltage upon being activated by the fuel cell maximumpower search function.

The fuel cell optimum operating point tracking function comprises a fuelcell maximum power point judging-and-storing unit capable of monitoringthe output voltage upon being activated by said fuel cell maximum powersearch function, and judging the maximum power point of the outputvoltage of the fuel cell.

The fuel cell maximum power point judging-and-storing unit is configuredso as to judge a voltage under which the command voltage becomes maximumwithin a range of the successive detection as the maximum power point,when the maximum power point is successively detected despite thecommand voltage is varied.

The fuel cell optimum operating point tracking function comprises atimer unit having a specified voltage refreshing interval set therein,configured so as to measure the power state by clearing, after everyelapse of the specified voltage refreshing interval, the stored valuesof the fuel cell output voltage maximum point and the output voltagecorresponded thereto upon being activated by the fuel cell maximum powersearch function, and by varying the output voltage of the fuel cell upto as high as the maximum voltage for the maximum power point trackingcontrol.

The fuel cell optimum operating point tracking function comprises anoptimum operating point variation command unit capable of tracking theoptimum operating point, by monitoring the power state at the fuel celloutput power maximum point upon activation with the aid of the fuel cellmaximum power search function to thereby keep the power source operationconstant at a stable condition, and by effecting the maximum powermonitoring through additionally giving a minimal voltage change ataround the current operating voltage value.

The fuel cell optimum operating point tracking system further comprisesan intermittent operation preventive function capable of monitoring,during the operation of the power source device, the output voltage ofthe fuel cell, and of outputting an output interruption voltage, uponlowering of the fuel cell output voltage to as low as the fuel celloutput interruption voltage or below, to thereby turn the power sourcedevice into interruption state.

The intermittent operation preventive function comprises a fuel celloutput voltage measuring unit capable of measuring the fuel cell outputvoltage during operation of the fuel cell optimum operating pointtracking function.

The intermittent operation preventive function comprises a fuel celloutput interruption judging unit capable of judging whether output ofthe fuel cell should be interrupted or not, when the fuel cell outputvoltage drops to the fuel cell output interruption voltage or belowduring operation of the fuel cell optimum operating point trackingfunction.

The intermittent operation preventive function comprises a timer unitcapable of controlling the intermittent operation when output of thefuel cell is interrupted during operation of the fuel cell optimumoperating point tracking function.

The timer unit is configured so as to interrupt the fuel cell, to set arestart wait time, to measure the output voltage of the fuel cell afterthe elapse of the restart wait time, and to output an operation outputsignal if the voltage reaches or exceeds the restart voltage to therebyactivate the fuel cell.

The intermittent operation preventive function comprises a fuel celloutput start judging unit capable of judging whether restart of the fuelcell interrupted during the operation of the fuel cell optimum operatingpoint tracking function is allowable or not.

The fuel cell optimum operating point tracking system further comprisesa fuel cell optimum operating point tracking and retaining functioncapable of widening the specified voltage refreshing interval, whenvariation in the output voltage of the fuel cell falls below the amountof variation of set voltage within a predetermined time period.

The fuel cell optimum operating point tracking and retaining functioncomprises a fuel cell optimum operating point tracking unit having thefuel cell maximum power search function and the fuel cell optimumoperating point tracking function.

The fuel cell optimum operating point tracking and retaining functioncomprises a fuel cell output voltage control value variation judgingunit capable of judging whether the output voltage variation of the fuelcell falls below, or exceeding the amount of variation of set voltagewithin a predetermined time period.

The fuel cell optimum operating point tracking and retaining functioncomprises a timer unit capable of operating so as to activate the fuelcell optimum operating point tracking unit, by widening the specifiedvoltage refreshing interval, if the output voltage variation of the fuelcell falls below the amount of variation of set voltage within apredetermined time period, and by initializing the specified voltagerefreshing interval, if the output voltage variation of the fuel cellexceeds the amount of variation of set voltage within a predeterminedtime period.

The fuel cell optimum operating point tracking and retaining function isconfigured so as to set a reference unit time, to count the number oftimes the output voltage variation of the fuel cell falls below theamount of variation of set voltage within every reference unit time, andto determine the state of fall below the amount of variation of setvoltage within the predetermined time period by the fact that aspecified number of count has successively been met.

The fuel cell optimum operating point tracking and retaining functioncomprises a counter capable of setting the reference unit time, and ofcounting the fact that the output voltage variation of the fuel cellfalls below the amount of variation of set voltage within everyreference unit time.

According to the present invention, there is also provided a powersource unit comprising the fuel cell optimum operating point trackingsystem of the present invention, the fuel cell optimum operating pointtracking system being used in a power source device powered by a fuelcell, configured so as to improve the responsiveness thereof bymonitoring the power state while varying the output voltage of the fuelcell, and by allowing the fuel cell to start operation at an inputvoltage corresponded to a maximum power point thereof, comprising: afuel cell maximum power search function; and a fuel cell optimumoperating point tracking function capable of tracking an optimumoperating point through maximum power monitoring, by allowing the fuelcell maximum power search function to periodically operate so as tomonitor the power state to thereby keep the power source operationconstant at a stable condition, and additionally giving a minimalvoltage change at around the current operating voltage value.

According to the present invention, it is made possible to trackvariable operation voltage of a fuel cell capable of supplying a maximumoutput power, taking temperature changes and chemical reactions intoconsideration, by monitoring the power state while varying the outputvoltage of the fuel cell upon activation thereof, and by allowing thefuel cell to start operation at a voltage corresponded to the maximumpower point.

It is also made possible to realize a stable power supply from the fuelcell, by monitoring the power state while periodically varying theoutput voltage of the fuel cell, and by allowing the fuel cell to startoperation at a voltage corresponded to the maximum power point thereof.

By equipping the fuel cell maximum power search function with a powersource activation detecting unit capable of measuring the power statewhile varying the output voltage of the fuel cell upon activationthereof up to as high as the maximum voltage for the maximum power pointtracking control, it is no more necessary to clear a stored value of themaximum point of the fuel cell output power and the output voltagecorresponded thereto upon activation of the fuel cell for everyspecified voltage refreshing interval, and to measure the power statewhile varying the output voltage of the fuel cell up to as high as themaximum voltage for the maximum power point tracking control, and it ismade possible to measure the operation point to be defined as themaximum power point even under a power restriction by a load.

By providing the intermittent operation preventive function capable ofmonitoring the fuel cell output voltage during operation of the powersource device, and of turning the fuel cell into an interruption stateby outputting an interruption output signal when the voltage falls belowthe fuel cell output interruption voltage, it is made possible tocontrol the duration of time of the intermittent oscillation, and torealize a stable power supply from the fuel cell.

Because the setting is made so as to widen the specified voltagerefreshing interval when the output voltage variation of the fuel cellfalls below the amount of variation of set voltage within apredetermined time period, it is also made possible to stabilize theoperation state even if the number of times of the maximum operationpoint tracking is reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, advantages and features of the presentinvention will be more apparent from the following description taken inconjunction with the accompanying drawings, in which:

FIG. 1 is a block diagram showing a best mode of a power source devicefor carrying out the present invention;

FIG. 2 is a flow chart showing processing of the fuel cell maximum powersearch function;

FIG. 3 is an operational waveform chart in the fuel cell maximum powersearch function;

FIG. 4 is again an operational waveform chart in the fuel cell maximumpower search function;

FIG. 5 is again an operational waveform chart in the fuel cell maximumpower search function;

FIG. 6 is again an operational waveform chart in the fuel cell maximumpower search function;

FIG. 7 is again an operational waveform chart in the fuel cell maximumpower search function;

FIG. 8 is again an operational waveform chart in the fuel cell maximumpower search function;

FIG. 9 is again an operational waveform chart in the fuel cell maximumpower search function;

FIG. 10 is again an operational waveform chart in the fuel cell maximumpower search function;

FIG. 11 is a block diagram showing an embodiment having a fuel cellmaximum power search function different from that in the embodimentshown in FIG. 1;

FIG. 12 is a flow chart of the embodiment shown in FIG. 11;

FIG. 13 is a flow chart showing processing of a fuel cell optimumoperating point tracking function;

FIG. 14 is an operational waveform chart in the fuel cell optimumoperating point tracking function;

FIG. 15 is a block diagram showing a best mode for carrying out anintermittent oscillation preventive operation according to the presentinvention;

FIG. 16 is a flow chart showing processing of the embodiment shown inFIG. 15;

FIG. 17 is again a flow chart showing processing of the embodiment shownin FIG. 15;

FIG. 18 is a block diagram showing a best mode for carrying out anoptimum operating point tracking and retaining operation according tothe present invention;

FIG. 19 is a flow chart showing processing of the embodiment shown inFIG. 18; and

FIG. 20 is a circuit diagram of a conventional power source deviceequipped with an operating point tracking circuit.

EXPLANATION OF THE MARKS

-   1 fuel cell-   2 converter-   3 load-   11 fuel cell output voltage variation command circuit-   12 fuel cell output power measuring circuit-   13 fuel cell output power maximum power point judging and storing    circuit-   14 timer circuit-   15 operation point variation command circuit-   16 power source activation detecting circuit-   21 fuel cell output voltage measuring circuit-   22 fuel cell output interruption judging circuit-   23 timer circuit-   24 fuel cell output start judging circuit-   31 optimum operation point tracking circuit-   32 fuel cell output voltage control value variation detecting    circuit-   33 counter-   34 timer circuit

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A block diagram of a best mode for carrying out the present invention isshown in FIG. 1. A power source device is shown in FIG. 1 as an examplewherein a DC-DC converter 2 is used as the power source device connectedto a fuel cell 1. The power source device has a load 3 connected on theoutput side thereof. The power source device has a fuel cell optimumoperating point tracking system, and connected to a converter 2.

The fuel cell optimum operating point tracking system comprises a fuelcell maximum power search function and a fuel cell optimum operatingpoint tracking function. A specific configuration for accomplishingthese functions is shown in FIG. 1, leaving a detail thereof for laterdescription.

The fuel cell optimum operating point tracking system has a fuel celloutput voltage shift command circuit 11 capable of varying voltageoutput by the fuel cell 1 upon activation of the converter 2 to amaximum voltage for the maximum power point tracking control. The fuelcell output voltage shift command circuit 11 is configured so as to seta maximum command voltage and a minimum command voltage, vary an initialcommand voltage output upon activation up to the maximum commandvoltage, and then lower the voltage from the maximum command voltagedown to the minimum command voltage.

The fuel cell optimum operating point tracking system has a fuel celloutput power measuring circuit 12 capable of measuring a power state asa result of output voltage variation by the fuel cell output voltageshift command circuit 11.

The fuel cell optimum operating point tracking system has a fuel cellmaximum power point judging and storing circuit 13 capable of judging amaximum power point of the output power of the fuel cell, by monitoringthe power state as a result of output voltage variation by the fuel celloutput voltage shift command circuit 11, and by monitoring also theoutput voltage measured by the fuel cell output power measuring circuit12.

The fuel cell maximum power point judging and storing circuit 13 isconfigured so as to judge a voltage under which the command voltagebecomes maximum within a range of the successive detection as themaximum power point, when the maximum power point is successivelydetected despite the output voltage is varied by the fuel cell outputvoltage shift command circuit 11.

The fuel cell optimum operating point tracking system has a timercircuit 14 capable of periodically activating the fuel cell outputvoltage shift command circuit 11 and the fuel cell output powermeasuring circuit 12. The timer circuit 14 has a specified voltagerefreshing interval set therein, and is configured so as to measure thepower state by clearing, after every elapse of the specified voltagerefreshing interval, the stored values of the fuel cell output voltagemaximum point and the output voltage of the fuel cell maximum powerpoint judging and storing circuit 13, and by varying the output voltageof the fuel cell up to as high as the maximum voltage for the maximumpower point tracking control through activation of the fuel cell outputvoltage shift command circuit 11 and the fuel cell output powermeasuring circuit 12.

The fuel cell optimum operating point tracking system has an optimumoperating point shift command circuit 15 capable of tracking the optimumoperating point, upon being instructed by the fuel cell output voltageshift command circuit 11 that variation in the output voltage hasfinished, by monitoring the power state with the aid of the fuel cellmaximum power point judging and storing circuit 13 to thereby keep thepower source operation constant at a stable condition, and by effectingthe maximum power monitoring through additionally giving a minimalvoltage change at around the current operating voltage value.

Operation and processing of the fuel cell maximum power search functionof thus-configured fuel cell optimum operating point tracking systemwill be explained referring to the flow chart shown in FIG. 2. Operationwaveform charts expressing the fuel cell maximum power search uponactivation are shown in FIG. 3 and FIG. 4, and operation waveform chartsexpressing the fuel cell maximum power search during normal operationare shown in FIG. 5 to FIG. 10.

When the power source is activated (S1), the timer circuit 14 turns on,and clears a stored value of the fuel cell output voltage maximum point(S2). At the same time, a stored value of the fuel cell output voltagecorresponded to the fuel cell output voltage maximum point is alsocleared (S3).

Next, the timer circuit 14 turns on, and the fuel cell output voltageshift command circuit 11 starts to operate, so as to vary the fuel celloutput voltage (S4). Thus-varied fuel cell output power is measured bythe fuel cell output power measuring circuit 12 (S5). Whetherthus-measured fuel cell output power is larger than a stored value ofthe fuel cell output voltage maximum point or not is judged by themaximum power point judging and storing circuit 13 (S6).

If the measured fuel cell output power was judged as being smaller thanthe stored value of the fuel cell output voltage maximum point, the fuelcell output power is stored as the fuel cell output voltage maximumpoint by the fuel cell maximum power point judging and storing circuit13 (S7).

For the case where the maximum power point was successively detectedeven if the output voltage of the fuel cell was varied by the fuel celloutput voltage shift command circuit 11 as shown in the waveform chartin FIG. 4A, a voltage under which the command voltage becomes maximumwithin a range of the successive detection is judged as the maximumpower point. The fuel cell output voltage of the fuel cell outputvoltage maximum point is also stored by the fuel cell maximum powerpoint judging and storing circuit 13 (S8), and whether the variation ofthe fuel cell output voltage has finished or not is judged (S9). If thevariation of the fuel cell output voltage has finished, the fuel celloutput voltage is shifted to the maximum power stored point (S10), andthereby the operation and processing of the fuel cell maximum powersearch come to the end (S11).

On the contrary, when the fuel cell output power measured at point “a+1”is judged as being smaller than the stored value of the fuel cell outputvoltage maximum point measured at point “a”, as shown in the waveformchart in FIG. 3, whether the variation of the fuel cell output voltagehas finished or not is judged (S9). If the variation of the fuel celloutput voltage has finished, the fuel cell output voltage is shifted tothe maximum power stored point (S10), and thereby the operation andprocessing of the fuel cell maximum power search come to the end (S11).

If the variation of the fuel cell output voltage has not ended, the fuelcell output voltage shift command circuit 11 reactivates, and makesanother variation in the fuel cell output voltage (S4). Thus-varied fuelcell output power is measured by the fuel cell output power measuringcircuit 12 (S5). On the contrary, if the variation of the fuel celloutput voltage has finished, the fuel cell output voltage is shifted tothe maximum power stored point (S10), and thereby the operation andprocessing of the fuel cell maximum power search come to the end (S11).

As has been described in the above and indicated by FIG. 3 and FIG. 4,the response characteristic was successfully improved by monitoring thepower state under varied voltage of the cell upon activation, and bystarting the operation at a voltage corresponded to the maximum powerpoint.

In this embodiment, the fuel cell maximum power search function isallowed to operate also in the normal operation, so as to monitor thepower state, as being periodically activated by the timer circuit 14. Asshown in FIG. 5, if there is no variation made in the maximum powerpoint, and the fuel cell output power measured at point “c+1” is judgedas being smaller than the stored value of fuel cell output voltagemaximum point measured at point “c”, this point “c”, judged as being thestored value, is kept unchanged as the operation point. On the otherhand, if the maximum power point was successively detected even if theoutput voltage was varied as shown in FIG. 6, a voltage under which thecommand voltage becomes maximum within a range of the successivedetection is judged as the maximum power point.

As shown in FIG. 7, when the load was found to be larger than the powergeneration capacity of the fuel cell, and the power generation capacityof the fuel cell was then increased, the fuel cell output power measuredat point “c+1” becomes larger than the fuel cell output power measuredat point “c”, defining the point “c+1” as the operation point. On thecontrary, if as shown in FIG. 8, the power generation capacity of thefuel cell was reduced, the fuel cell output power measured at point “c”is now larger than the fuel cell output power measured at point “c−1”,so that the point “c−1” is defined as the operation point.

As shown in FIG. 9, when the load was found to be smaller than the powergeneration capacity of the fuel cell, and the load was increased, thefuel cell output power measured at point “c+1” becomes larger than thefuel cell output power measured at point “c”, defining the point “c+1”as the operation point. On the contrary, if, as shown in FIG. 10, theload was reduced, the fuel cell output power measured at point “c” isnow larger than the fuel cell output power measured at point “c−1”, sothat the point “c−1” is defined as the operation point.

As has been described in the above and indicated by FIG. 5 to FIG. 10, astable operation can be ensured by monitoring of the power state withthe aid of the periodical activation of the timer circuit 14. In theperiodical operation with the aid of the timer circuit 14, the operationand processing shown in the flow chart in FIG. 2 can proceed, similarlyto the case when the power source is activated.

In this embodiment, the setting is made so that, for the case when themaximum power point was successively detected even if the output voltagewas varied by the fuel cell output voltage shift command circuit 11, avoltage under which the command voltage becomes maximum within a rangeof the successive detection is judged as the maximum power point. Thiswould be the best, but it is also allowable to make the setting so as tojudge the center voltage within a range of the successive detection asthe maximum power point, provided that the range of the successivedetection can be defined. The same will apply to the description below.

Next paragraphs will describe an embodiment having a fuel cell maximumpower search function different from that shown in FIG. 1. Specificconfiguration is shown in FIG. 11. The fuel cell optimum operating pointtracking system comprises, similarly to the above-described embodiment,the fuel cell output voltage shift command circuit 11 capable ofshifting a voltage output from the fuel cell 1 upon activation of theconverter 2 up to the maximum voltage for the maximum power pointtracking control; the fuel cell output power measuring circuit 12capable of measuring a power state as a result of output voltagevariation by the fuel cell output voltage shift command circuit 11; thefuel cell maximum power point judging and storing circuit 13 capable ofjudging a maximum power point of the output power of the fuel cell, bymonitoring the power state as a result of output voltage variation bythe fuel cell output voltage shift command circuit 11, and by monitoringalso the output voltage measured by the fuel cell output power measuringcircuit 12; and the optimum operating point shift command circuit 15capable of tracking the optimum operating point, upon being instructedby the fuel cell output voltage shift command circuit 11 that variationin the output voltage has finished, by monitoring the power state withthe aid of the fuel cell maximum power point judging and storing circuit13 to thereby keep the power source operation constant at a stablecondition, and by effecting the maximum power monitoring throughadditionally giving a minimal voltage change at around the currentoperating voltage value.

A principal feature of this embodiment resides in that a power sourceactivation detecting circuit 16 is provided in place of the timercircuit 14. The power source activation detecting circuit 16 isconfigured so as to measure the power state by varying the outputvoltage of the fuel cell upon activation thereof up to as high as themaximum voltage for the maximum power point tracking control.

Operation and processing of the fuel cell maximum power search functionof thus-configured fuel cell optimum operating point tracking systemwill be explained referring to the flow chart shown in FIG. 12.

When the power source is activated (S101), the power source activationdetecting circuit 16 turns on, and clears a stored value of the fuelcell output power (S102). Next, the power source activation detectingcircuit 16 turns on, and the fuel cell output voltage shift commandcircuit 11 starts to operate, so as to vary the fuel cell output voltage(S103). Thus-varied fuel cell output power is measured by the fuel celloutput power measuring circuit 12 (S104). Whether thus-measured fuelcell output power is larger than the previously-measured fuel celloutput power or not is judged by the maximum power point judging andstoring circuit 13 (S105).

If the measured fuel cell output power was judged as being smaller thanthe previously-measured fuel cell output power, the previously-measuredfuel cell output power voltage point is stored as the fuel cell outputvoltage maximum point by the fuel cell maximum power point judging andstoring circuit 13 (S106).

As shown in the waveform chart in FIG. 4, if the maximum power point wassuccessively detected even when the output voltage of the fuel cell wasvaried by the fuel cell output voltage shift command circuit 11, thepreviously-measured fuel cell output power voltage point, which is point“b” in FIG. 4, is assumed as the maximum power point, defined as theoperation point for the succeeding operations, and thereby the operationand processing of the fuel cell maximum power search come to the end(S109).

On the contrary, when the fuel cell output power measured at point “a+1”is judged as being smaller than the stored value of the fuel cell outputvoltage maximum point measured at point “a”, as shown in the waveformchart in FIG. 3, the point “a” is assumed as the maximum power point,defined as the operation point for the succeeding operations, andthereby the operation and processing of the fuel cell maximum powersearch come to the end (S109).

Whether the measured fuel cell output power is larger than thepreviously-measured fuel cell output power or not is judged by themaximum power point judging and storing circuit 13 (S105), and ifthus-measured fuel cell output power was judged as being smaller thanthe previously-measured fuel cell output power, the next judgment ismade on whether variation in the fuel cell output voltage has finishedor not (S107). If it was judged that the variation in the fuel celloutput voltage has not finished, the measured fuel cell output power isstored as the previously-measured fuel cell output power value (S108),and the fuel cell output voltage shift command circuit 11 activatesagain so as to vary the fuel cell output voltage (S103). Thus-variedfuel cell output power is measured by the fuel cell output powermeasuring circuit 12 (S104). On the contrary, if it was judged that thevariation of the fuel cell output voltage has finished, the operationand processing of the fuel cell maximum power search come to the end(S109).

As has been described in the above and indicated by FIG. 3 and FIG. 4,the response characteristic was successfully improved by monitoring thepower state under varied voltage of the cell upon activation, and bystarting the operation at a voltage corresponded to the maximum powerpoint.

In this embodiment, the fuel cell maximum power search function isallowed to operate also in the normal operation, so as to monitor thepower state, as being periodically activated by the power sourceactivation detecting circuit 16. As shown in FIG. 5, if there is novariation made in the maximum power point, and the fuel cell outputpower measured at point “c+1” is judged as being smaller than the storedvalue of fuel cell output voltage maximum point measured at point “c”,this point “c”, judged as being the stored value of fuel cell outputvoltage maximum point, is kept unchanged as the operation point. On theother hand, if the maximum power point was successively detected even ifthe output voltage was varied as shown in FIG. 6, thepreviously-measured fuel cell output power point, which is point “c” inFIG. 6, is assumed as the maximum power point, and defined as theoperation point for the succeeding operations.

As shown in FIG. 7, when the load was found to be larger than the powergeneration capacity of the fuel cell, and the power generation capacityof the fuel cell was then increased, the fuel cell output power measuredat point “c+1” becomes larger than the fuel cell output power measuredat point “c”, defining the point “c+1” as the operation point. On thecontrary, if as shown in FIG. 8, the power generation capacity of thefuel cell was reduced, the fuel cell output power measured at point “c”is larger than the fuel cell output power measured at point “c−1”, sothat the point “c−1” is defined as the operation point.

As shown in FIG. 9, when the load was found to be smaller than the powergeneration capacity of the fuel cell, and the load was increased, thefuel cell output power measured at point “c+1” becomes larger than thefuel cell output power measured at point “c”, defining the point “c+1”as the operation point. On the contrary, if, as shown in FIG. 10, theload was reduced, the fuel cell output power measured at point “c” islarger than the fuel cell output power measured at point “c−1”, so thatthe point “c−1” is defined as the operation point.

As is obvious from the above, by equipping the fuel cell maximum powersearch function with the power source activation detecting circuit 16capable of measuring the power state while varying the output voltage ofthe fuel cell upon activation thereof up to as high as the maximumvoltage for the maximum power point tracking control, it is no morenecessary to clear stored values of the maximum point of the fuel celloutput power and the output voltage corresponded thereto upon activationof the fuel cell for every specified voltage refreshing interval, and tomeasure the power state while varying the output voltage of the fuelcell up to as high as the maximum voltage for the maximum power pointtracking control, and it is made possible to measure the operation pointto be defined as the maximum power point even under a power restrictionby a load.

Although the power source activation detecting circuit 16 was providedin this embodiment, it is also allowable to provide the timer circuit14. For the case where the timer circuit 14 is provided, it ispreferable to, similarly to as the foregoing embodiment, periodicallyactivate the fuel cell output voltage shift command circuit 11 and thefuel cell output power measuring circuit 12, and to vary the outputvoltage of the fuel cell up to as high as the maximum voltage for themaximum power point tracking control, to thereby measure the powerstate. In the periodical operation with the aid of the timer circuit 14,the operation and processing shown in the flow chart in FIG. 12 canproceed, similarly to the case when the power source is activated.

Next paragraphs will describe operation and processing of the fuel celloptimum operating point tracking function capable of tracking theoptimum operating point by effecting the maximum power monitoringthrough additionally giving a minimal voltage change at around thecurrent operating voltage value, referring to the flow chart shown inFIG. 13. Operation waveform of the operation and processing of theoptimum operating point are shown in FIG. 14.

Upon completion of the operation and processing of the fuel cell maximumpower search, timer count-up starts (S21), the timer circuit 14 turnson, and clears a stored value of the fuel cell output voltage maximumpoint (S22). At the same time, a stored value of the fuel cell outputvoltage corresponded to the fuel cell output voltage maximum point isalso cleared (S23).

Next, the timer circuit 14 activates the fuel cell output voltage shiftcommand circuit 11 and the fuel cell output power measuring circuit 12,allows the fuel cell output power measuring circuit 12 to measure thecurrent fuel cell output power, and allows the maximum power pointjudging and storing circuit 13 to store the current fuel cell outputpower (S24). Next as shown in FIG. 14, the maximum power point judgingand storing circuit 13 stores the fuel cell output voltage as Vn (S25),and allows the fuel cell output voltage shift command circuit 11 toshift the fuel cell output voltage to (Vn+ΔVn) (S26).

Next, the shifted fuel cell output power is measured by the fuel celloutput power measuring circuit 12, and the fuel cell output power isstored by the maximum power point judging and storing circuit 13 (S27).Next, as shown in FIG. 14, the fuel cell output voltage is shifted to(Vn−ΔVn) by the fuel cell output voltage shift command circuit 11 (S28),the shifted fuel cell output power is measured by the fuel cell outputpower measuring circuit 12, and the fuel cell output power is stored bythe maximum power point judging and storing circuit 13 (S29).

The fuel cell output voltage is then shifted to the fuel cell outputvoltage maximum point in the fuel cell output power measured in stepsS24, S27 and S29, or shifted to a point where the command output voltageis maximized when a plurality of maximum power points were observed(S30), and thereby the processing comes to the end (S31). Repetition ofthis procedure makes it possible to monitor the maximum power and totrack the optimum operation point, while keeping the power sourceoperation constant at a stable condition.

The succeeding interruption of the power supply by the fuel cell raisesthe fuel cell voltage, and this may falsely be judged as operationenable, and may result in an intermittent operation. The fuel celloptimum operating point tracking system of the present invention istherefore provided with the intermittent operation preventive functioncapable of monitoring the fuel cell output voltage during operation ofthe power source device, and of turning the fuel cell into aninterruption state by outputting an interruption output signal when thefuel cell output voltage falls to or below the fuel cell outputinterruption voltage. A block diagram of the power source deviceequipped with the intermittent operation preventive function is shown inFIG. 15.

The intermittent operation preventive function has a fuel cell outputvoltage measuring circuit 21 allowing the converter 2 to measure thefuel cell output voltage during operation of the fuel cell. Theintermittent operation preventive function also has a fuel cell outputinterruption judging circuit 22 capable of judging whether the output ofthe fuel cell 1 should be interrupted or not, when the fuel cell outputvoltage measured by the fuel cell output voltage measuring circuit 21falls to or below the fuel cell output interruption voltage. The fuelcell output interruption judging circuit 22 is configured so as togenerate an output interruption signal to the converter 2, wheninterruption of the fuel cell 1 was judged.

The intermittent operation preventive function has a timer circuit 23capable of controlling intervals of the intermittent oscillation of thefuel cell 1. The timer circuit 23 is configured so as to turn theconverter 2 into an interruption state, to set a restart wait time, andto measure the output voltage of the fuel cell 1 after the elapse of therestart wait time.

There is also provided a fuel cell output start judging circuit 24 towhich data of the output voltage of the fuel cell 1 measured by the fuelcell output voltage measuring circuit 21 is entered after the elapse ofthe restart wait time set by the timer circuit 23. The fuel cell outputstart judging circuit 24 is configured so as to judge whether the outputvoltage of the fuel cell 1 after the elapse of the restart wait time islarge enough to restart the fuel cell 1 or not, and to output anoperation output signal to thereby bring the converter 2 into operationstate, when the output voltage was judged as having reached or exceededthe restart voltage of the fuel cell 1.

Operation and processing for preventing the intermittent operation ofthus-configured fuel cell optimum operating point tracking system willbe explained referring to the flow chart shown in FIG. 16. When the fuelcell stops the power supply, raises the voltage and is judged as beingoperable, the intermittent operation begins (S41). This allows the fuelcell output voltage measuring circuit 21 to measure the output voltageof the fuel cell 1 (S42). Whether the output of the fuel cell 1 isinterrupted or not is judged by the fuel cell output interruptionjudging circuit 22 (S43).

Next, when the output of the fuel cell 1 was judged as being notinterrupted, whether the fuel cell output voltage measured by the fuelcell output voltage measuring circuit 21 is equal to or lower than thefuel cell output interruption voltage or not is judged by the fuel celloutput interruption judging circuit 22 (S44). When the fuel cell outputvoltage is judged as being equal to or smaller than the fuel cell outputinterruption voltage, a fuel cell output interruption signal is sent tothe converter 2 (S45), also to the timer circuit 23, to thereby starttimer count for the intermittent operation (S46), and the process comesto the end (S51).

FIG. 17 is a flow chart showing operation and processing proceeded afterthe intermittent operation timer count is started. Upon start of theintermittent operation timer count (S61), the intermittent operationtimer count is conducted by the timer circuit 23 (S62). Whether thetimer count-up has reached or not is judged by the timer circuit 23(S63), and confirmation of the timer count-up leads the process ofintermittent timer count to the end (S64). By this procedure, it is madepossible to turn the converter 2 into the interruption state by sendingan interruption output signal, when the fuel cell output voltage fallsto or below the fuel cell output interruption voltage during operationof the converter 2.

On the other hand, when the fuel cell output voltage was judged as beingequal to or exceeding the fuel cell output interruption voltage, whetherthe intermittent operation timer count is in process or not is judged(S47), and if the intermittent operation timer count was judged as beingnot in process, a judgment is then made on whether the fuel cell 1 is inoutput operation (S48). If the fuel cell 1 was judged as being not inoutput operation, a judgment is made again on whether the fuel celloutput voltage is equal to or exceeds the fuel cell output interruptionvoltage or not (S49), and if the fuel cell output voltage is equal to orexceeds the fuel cell output interruption voltage, the output operationof the fuel cell 1 is started (S50), and thereby the operation andprocessing for preventing the intermittent operation come to the end(S51).

The fuel cell optimum operating point tracking system of the presentinvention further comprises an optimum operating point tracking andretaining function capable of widening the specified voltage refreshinginterval, when variation in the output voltage of the fuel cell 1 fallsbelow the amount of variation of set voltage within a predetermined timeperiod. The optimum operating point tracking and retaining function isset so that it can recover the original specified voltage refreshinginterval, when variation in the output voltage was found to exceed theamount of variation of set voltage after the widening of the specifiedvoltage refreshing interval. FIG. 18 is a block diagram of a powersource device provided with the optimum operating point tracking andretaining function.

The optimum operating point tracking and retaining function isconfigured as having optimum operating point tracking circuit 31tracking the optimum operating point. The optimum operating pointtracking circuit 31 further comprises a fuel cell maximum power searchfunction and a fuel cell optimum operating point tracking function, andmost preferably has a circuit configuration almost same as that of thefuel cell optimum operating point tracking system shown in FIG. 1.

The optimum operating point tracking circuit 31 tracks the optimumoperating point, and sends an optimum operating point trackinginformation to the converter 2. The optimum operating point tracking andretaining function has a fuel cell output voltage control valuevariation detecting circuit 32. The fuel cell output voltage controlvalue variation detecting circuit 32 is configured to receive theoptimum operating point tracking information from the optimum operatingpoint tracking circuit 31, to detect variation in the control value, andto judge whether the variation in the output voltage of the fuel cellfell below or exceeded the amount of variation of set voltage within apredetermined time period.

The optimum operating point retaining unit has a counter 33, and isconfigured to activate counting by the counter 33, when the variation inthe output voltage is judged, by the fuel cell output voltage controlvalue variation detecting circuit 32, as having fallen below the amountof variation of set voltage.

The optimum operating point retaining unit has a timer circuit 34. Ifthe variation in the output voltage detected by the fuel cell outputvoltage control value variation detecting circuit 32 fell below theamount of variation of set voltage for a predetermined period, and apredetermined number of times of this event was successively counted bythe counter, it is judged that the output voltage fell below the amountof variation of set voltage within the predetermined time period, andthe specified voltage refreshing interval is widened. On the contrary,if the amount of variation of set voltage detected by the fuel celloutput voltage control value variation detecting circuit 32 exceeded theamount of variation of set voltage, the specified voltage refreshinginterval is initialized.

Next paragraphs will describe operation and processing of the fuel celloptimum operating point retention in thus-configured fuel cell optimumoperating point tracking system, referring to the flow chart shown inFIG. 19. Upon start of the retention operation of the fuel cell optimumoperating point tracking (S71), the optimum operating point is trackedby the optimum operating point tracking circuit 31 (S72).

An optimum operating point information tracked by the optimum operatingpoint tracking circuit 31 is sent to the converter 2, and also to thefuel cell output voltage control value variation detecting circuit 32.The fuel cell output voltage control value variation detecting circuit32, which received the optimum operating point information, then detectsvariation in the control value, and judges whether the variation in thecontrol value for the fuel cell output voltage exceeds or falls belowthe amount of variation of the set voltage over a predetermined timeperiod (S73).

If the variation in the control value for the fuel cell output voltagefalls below the amount of variation of set voltage over a predeterminedtime period, the fuel cell output voltage control value variationdetecting circuit 32 makes a judgment of no variation, and theinformation is sent to the counter 33, with which the number of times ofno variation in the fuel cell output voltage is counted (S75).

Next, whether the number of counts of no variation equals to or exceedsthe predetermined number of times or not is judged (S76). If thepredetermined number is not met, operation and processing of fuel celloptimum operating point retention once comes to the end (S79), and theoperation and processing of fuel cell optimum operating point retentionis restarted (S71).

On the other hand, if the number of counts of no variation counted bythe counter equals to or exceeds the predetermined number of times, thisinformation is set to the timer circuit 34, and the specified voltagerefreshing interval is widened (S77). The timer circuit 34 clears thenumber of times of no variation in the fuel cell output voltage (S78).By this procedure, the operation and processing of the fuel cell optimumoperating point retention once comes to the end (S79), and the operationand processing of fuel cell optimum operating point retention arerestarted (S71). Widening of the specified voltage refreshing intervalas described in the above makes it possible to reduce the number oftimes of the maximum operation point tracking, and to stabilize theoperation conditions.

On the contrary, when a judgment is made on whether the variation in thecontrol value for the fuel cell output voltage exceeds or falls belowthe amount of variation of set voltage over a predetermined time period(S73), and if the variation in the control value for the fuel celloutput voltage exceeds the amount of variation of set voltage over thepredetermined time period, the fuel cell output voltage control valuevariation detecting circuit 32 makes a judgment of “variation observed”.

The information is sent to the timer circuit 34, and the operation andprocessing of the fuel cell optimum operating point retention areinitialized by the timer circuit 34 (S74). More specifically, theoriginal state is recovered if the operation and processing of the fuelcell optimum operating point retention has widened the specified voltagerefreshing interval. By this procedure, the operation and processing ofthe fuel cell optimum operating point retention once come to the end(S79), and the operation and processing of fuel cell optimum operatingpoint retention are restarted (S71).

It is preferable that each of the timer circuits 14, 23 and 34 in theabove-described embodiments is configured so as to generate a referenceclock, and so as to be synchronized with the reference clock.

Although each of the power source devices described in the foregoingembodiments was exemplified by a DC-DC converter, the present inventioncan be applicable also to any other power source devices, such as thoseusing a DC-AC inverter, for example.

Although each of the fuel cell optimum operating point tracking systemin the foregoing embodiments was described on the assumption ofmicrocomputer, such system can be configured as being incorporated intoother unit, or circuit, for example.

INDUSTRIAL APPLICABILITY

According to the present invention, it is made possible to trackvariable operation voltage of a fuel cell capable of supplying a maximumoutput power, taking temperature changes and chemical reactions intoconsideration, by monitoring the power state while varying the outputvoltage of the fuel cell upon activation thereof, and by allowing thefuel cell to start operation at a voltage corresponded to the maximumpower point.

It is also made possible to realize a stable power supply from the fuelcell, by monitoring the power state while periodically varying theoutput voltage of the fuel cell, and by allowing the fuel cell to startoperation at a voltage corresponded to the maximum power point thereof.

By equipping the fuel cell maximum power search function with a powersource activation detecting unit capable of measuring the power statewhile varying the output voltage of the fuel cell upon activationthereof up to as high as the maximum voltage for the maximum power pointtracking control, it is no more necessary to clear a stored value of themaximum point of the fuel cell output power and the output voltagecorresponded thereto upon activation of the fuel cell for everyspecified voltage refreshing interval, and to measure the power statewhile varying the output voltage of the fuel cell up to as high as themaximum voltage for the maximum power point tracking control, and it ismade possible to measure the operation point to be defined as themaximum power point even under a power restriction by a load.

By providing the intermittent operation preventive function capable ofmonitoring the fuel cell output voltage during operation of the powersource device, and of turning the fuel cell into an interruption stateby outputting an interruption output signal when the voltage falls to orbelow the fuel cell output interruption voltage, it is made possible tocontrol the duration of time of the intermittent oscillation, and torealize a stable power supply from the fuel cell.

Because the setting is made so as to widen the specified voltagerefreshing interval when the output voltage variation of the fuel cellfalls below the amount of variation of set voltage within apredetermined time period, it is also made possible to stabilize theoperation state even if the number of times of the maximum operationpoint tracking is reduced.

It is apparent that the present invention is not limited to the aboveembodiments, that may be modified and changed without departing from thescope and spirit of the invention.

1. A fuel cell optimum operating point tracking system used in a powersource device powered by a fuel cell, configured so as to improve aresponsiveness thereof by monitoring a power state, comprising: a fuelcell output power measuring unit capable of measuring the power state ofthe fuel cell; and a fuel cell optimum operating point tracking unitcapable of tracking an optimum operating point, so that a power sourceoperation is kept constant at a stable condition, wherein the fuel celloptimum operating point tracking unit finds the optimum operating pointby changing the power state of the fuel cell by a minimal value fromaround a current operating state.
 2. The fuel cell optimum operatingpoint tracking system as claimed in claim 1, wherein the power state isan output voltage of said fuel cell, and the optimum operating point isa maximum power point of the fuel cell, and said fuel cell optimumoperating point tracking unit comprises a fuel cell output voltagevariation command unit capable of varying the output voltage of saidfuel cell upon activation thereof up to as high as maximum voltage formaximum power point tracking control.
 3. The fuel cell optimum operatingpoint tracking system as claimed in claim 1, wherein the power state isan output voltage of said fuel cell, and the fuel cell output powermeasuring unit measures the output voltage of said fuel cell as thepower state while varying the output voltage upon activation thereof. 4.The fuel cell optimum operating point tracking system as claimed inclaim 1, wherein said fuel cell optimum operating point tracking unitcomprises a fuel cell maximum power point judging-and-storing unitcapable of monitoring an output power of said fuel cell upon activationthereof, and of judging the maximum power point of the output voltage ofsaid fuel cell.
 5. The fuel cell optimum operating point tracking systemas claimed in claim 4, wherein said fuel cell maximum power pointjudging-and-storing unit is configured so as to judge the output powerbecomes a maximum in a successive detection as the maximum power point,when the maximum power point is successively detected while the outputvoltage is varied.
 6. The fuel cell optimum operating point trackingsystem as claimed in claim 1, wherein the power state is an outputvoltage of the fuel cell, and the optimum operating point is a maximumpower point of the fuel cell, and wherein said fuel cell optimumoperating point tracking unit comprises a timer having a specifiedvoltage refreshing interval set therein, configured so as to measure theoutput voltage of the fuel cell by clearing, after an elapse of saidspecified voltage refreshing interval, stored values of the maximumpower point and the output voltage corresponded thereto upon activationof said fuel cell, and by varying the output voltage of said fuel cellup to as high as maximum voltage for maximum power point trackingcontrol.
 7. The fuel cell optimum operating point tracking system asclaimed in claim 1, wherein the power state is an output voltage of saidfuel cell, and said fuel cell optimum operating point tracking unitcomprises a power source activation detecting unit capable of detectingan activation of the power source device, and wherein the fuel celloutput power measuring unit measures the output voltage as the powerstate, while varying the output voltage of the fuel cell upon theactivation thereof, up to as high as maximum voltage for the maximumpower point tracking control.
 8. The fuel cell optimum operating pointtracking system as claimed in claim 1, wherein said fuel cell optimumoperating point tracking unit comprises an optimum operating pointvariation command unit capable of tracking the optimum operation point,by monitoring the power state at a fuel cell output power maximum pointupon activation of said fuel cell, and by effecting the maximum powermonitoring by changing the power state of the fuel cell by a minimalvalue from around current operating state.
 9. The fuel cell optimumoperating point tracking system as claimed in claim 1, furthercomprising an intermittent operation preventive unit capable ofmonitoring, during an operation of said power source device, the outputvoltage of the fuel cell, and of outputting an output interruptionvoltage, upon lowering of the output voltage of the fuel cell to a fuelcell output interruption voltage or below, to thereby turn said powersource device into interruption state.
 10. The fuel cell optimumoperating point tracking system as claimed in claim 9, wherein saidintermittent operation preventive unit comprises a fuel cell outputinterruption judging unit capable of judging whether an output of thefuel cell is interrupted or not, when the output voltage of the fuelcell drops to the fuel cell output interruption voltage or below duringan operation of said fuel cell optimum operating point tracking unit.11. The fuel cell optimum operating point tracking system as claimed inclaim 9, wherein said intermittent operation preventive unit comprises atimer unit capable of controlling the intermittent operation when anoutput of the fuel cell is interrupted during operation of said fuelcell maximum power search unit.
 12. The fuel cell optimum operatingpoint tracking system as claimed in claim 11, wherein said timer unit isconfigured so as to interrupt said fuel cell, to set a restart waittime, to measure the output voltage of said fuel cell after an elapse ofthe restart wait time, and to output an operation output signal if theoutput voltage reaches or exceeds a restart voltage to thereby activatesaid fuel cell.
 13. The fuel cell optimum operating point trackingsystem as claimed in claim 9, wherein said intermittent operationpreventive unit comprises a fuel cell output start judging unit capableof judging whether restart of the operation of the fuel cell interruptedduring the operation of said fuel cell optimum operating point trackingunit is allowable or not.
 14. The fuel cell optimum operating pointtracking system as claimed in claim 1, further comprising a fuel celloptimum operating point tracking and retaining unit capable of wideninga specified voltage refreshing interval, when variation in the outputvoltage of said fuel cell falls below the amount of variation of setvoltage within a predetermined time period.
 15. The fuel cell optimumoperating point tracking system as claimed in claim 14, wherein saidfuel cell optimum operating point tracking and retaining unit comprisesa fuel cell output voltage control value variation judging unit capableof judging whether the output voltage variation of said fuel cell fallsbelow, or exceeding the amount of variation of set voltage within apredetermined time period.
 16. The fuel cell optimum operating pointtracking system as claimed in claim 14, wherein said fuel cell optimumoperating point tracking and retaining unit comprises a timer unitcapable of operating so as to activate said fuel cell optimum operatingpoint tracking unit, by widening the specified voltage refreshinginterval, if the output voltage variation of said fuel cell falls belowthe amount of variation of set voltage within a predetermined timeperiod, and by initializing said specified voltage refreshing interval,if the output voltage variation of said fuel cell exceeds the amount ofvariation of set voltage within a predetermined time period.
 17. Thefuel cell optimum operating point tracking system as claimed in claim14, wherein said fuel cell optimum operating point tracking andretaining unit is configured so as to set a reference unit time, tocount the number of times the output voltage variation of said fuel cellfalls below said amount of variation of set voltage within everyreference unit time, and to determine a state of fall below the amountof variation of set voltage within said predetermined time period by thefact that a specified number of count has successively been met.
 18. Thefuel cell optimum operating point tracking system as claimed in claim17, wherein said fuel cell optimum operating point tracking andretaining unit comprises a counter capable of setting the reference unittime, and of counting an event in which the output voltage variation ofsaid fuel cell falls below said amount of variation of set voltagewithin every reference unit time.
 19. A power source unit comprising thefuel cell optimum operating point tracking system as claimed in claim 1,wherein said fuel cell optimum operating point tracking system is usedin the power source device powered by the fuel cell, configured so as toimprove responsiveness thereof by monitoring the power state whilevarying the output voltage of said fuel cell, and by allowing said fuelcell to start an operation at an input voltage corresponded to a maximumpower point thereof, comprising: said fuel cell maximum power searchunit capable of tracking an optimum operating point through maximumpower monitoring, by allowing said fuel cell maximum power search unitto periodically operate so as to monitor the power state to thereby keepthe power source operation constant at a stable condition, andadditionally giving a minimal voltage change at around the currentoperating voltage value.